Shear Strength of Earthbag Wall With Weak Cohesive Fill

7/30/2019 Shear Strength of Earthbag Wall With Weak Cohesive Fill

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1 Shear Strength of Earthbag Wall with Weak Cohesive Fill

SHEAR STRENGTH OF EARTHBAG WALL WITH WEAK COHESIVE FILL

Patricia Stouter, August 2011

www.Simple Earth Structures.com, [email protected]

ABSTRACT

Earthbag is a growing sustainable building technique that is increasingly desired in the developing world

because it costs only a quarter as much as concrete block walls, is easy to learn, and does not require

power tools. Because other geo-textiles perform well in hazardous areas, earthbag is desired in seismic

risk areas. But structural testing is needed to determine safe plan and construction guidelines for

different levels of risk.

Earthbag is usually built with cohesive earthen fill. Because it is often designed to fulfill adobe building

standards, earthbag can be compared to adobe test and standards values to provide safe guidelines for

use in the developing world. But to date almost all research by engineers has been limited to thestructural performance of atypical sand-filled bag walls.

To explore the shear strength of a weak cohesive fill, a plastered earthbag wall portion was subjected to

static diagonal compression.

The earthen- and lime-plastered test wall was still stable after

surpassing peak strength. With no reinforcing mesh, its peak

shear strength was superior to most sand bag walls tested, and

comparable to one with cement stucco reinforced with chicken

wire. More rigorous testing of both weak and strong cohesive fills

in earthbags is merited.

INTRODUCTION

Earthbag building is a sustainable hybrid earth technology using non-biodegradable bags or tubes as

formwork for cohesive, moist earthen fill. A conventional earthbag is filled with a cohesive soil mix that

becomes firm when tamped and cured. It becomes a type of un-fired earthen masonry unit, similar to

adobe.

Earthbag actually comprises a family of geo-textile materials. Mesh tubing, fibrous fill, and sand or

gravel fill are different options. Base or plinth layers of sand or rounded gravel bags may also be able to

serve as simple vibration dampening or base isolation devices. But 90% of the earthbag building around

the world uses only cohesive soil fills in solid fabric for strength and ease of construction.

Soil fill is chosen and modified to fit very specific conditions and needs. Soil fills influence strength,

construction difficulty, wall cost, and usefulness. Cement stucco works best with sandy fills while lime

plaster matches the expansion and drying needs of clay-rich fills in most climates. But the potentially

strongest wall assemblies of the conventional construction techniques should be explored first.


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.

RESEARCH PRECEDENTS

Unfortunately, to date testing has mostly been done on atypical sand fills, as shown below.

Sands are less variable than soils, simpler to analyze and repeat tests. Unlike cohesive soils, they require

little curing time before testing. Filled bags can be reused for the next test. But because sandbags tend

to slump and do not grip barbed wire well, they often in practice require significant amounts of cement

that raise costs in thick walls.

The only study of a full-sized, plastered earthbag building portion to date is a U-shaped wall completed

at the Cape Peninsula University of Technology1 in South Africa for Dr. Johnny Anderton of Eternally

Solar.2 With a chicken wire reinforced cement stucco surface this sandbag wall proved strong enough for

moderate wind conditions in a region without seismic risk.

This wall system uses channelized sewn bags without barbed wire or vertical rebar that reduces labor. It

is well suited to remote sites or locations where metal may corrode, such as near coasts, volcanoes, or

industrial centers. It uses approximately 45% more bags per wall because courses are thinner.

The CPUT test was primarily for flexural strength, and showed a peak strength of 44 psf/ 2.1 kPa. Thepeak strength of the return walls in shear may not be truly representative, but appeared to be 23 psf/

1.1 kPa. This bag system should be also tested with cohesive earthen fill in a straight wall portion, with

and without added barbed wire to compare to standard earthbag strengths.

1Ryno Thiart, Stability of the Sandbag Wall System to Horizontal Load Specified by Agrment, unpublished

dissertation for Cape Peninsula University of Technology faculty of Engineering, 20082www.earthbagbuild.com

TYPES OF EARTHBAG FILL

CONVENTIONAL COHESIVE:COMMON USE NON-COHESIVE:LIMITED USE

TYPE OF

FILL

Strong

Fill:

>30% clay,

a lot of

sand, little

silt

Medium

Fill:

20% clay,

20% clay,

mostly silt

Stable Fill:

Gravel,

pumice,

rubble

Partially

Structural

Fill:

Sand

Non-

structural

Fill:Rice Hulls,

other soft fill

STRENGTH Highstrength

Good

strength

Good

strength

Low

strength

Often good

strength

Very low

strength

Very low

strength

BENEFITS/

LIMITA-

TIONS

Heavy

work to

build,

cementstucco

possible in

tropics

Easy to

build, can

have

cementstucco in

warm

regions

Easy to

build, can

have

cementstucco in all

climates

Option for

stable

plans in

non-hazardous

areas

Flood-

resistant,

pumice is

high R,

chopped

rubble for

disasters

Flood-

resistant,

difficult to

build, costly

reinforced

structural

skin

For cold

regions, pest

susceptible,

experimentalwith costly

structural skin
http://www.earthbagbuild.com/http://www.earthbagbuild.com/http://www.earthbagbuild.com/http://www.earthbagbuild.com/


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A test at the University of Florida used air motion to test an un-plastered sandbag wall. Preliminaryresults show the structure did not become unstable when loaded with 60 psf/ 2.9 kPa.3

The most complete studies to date of bag walls are by three different graduate students at Bath

University. Their small bags created walls approximately 80% the width of a conventional earthbag wall.4

Chris Croft reported on his and earlier tests of flexural and shear strength of these sand bags.5

Test results showed that earthbag walls with barbed wire can sustain considerable deformation without

collapse. Crofts work also showed that plaster covering both sides of the wall increased strength of the

wall assembly significantly. When cement stucco was used, the peak shear strength (without barbed

wire) was about 119 psf/ 5.7 kPa or 180 psf/ 8.6 kPa with chicken wire.

How do these tested strengths of sand-filled bags compare to other earthen building materials?

COMPARING EARTHBAG TO ADOBE

Earthbag builders have successfully used adobe guidelines in non-hazardous regions for decades. Like

adobe, bag walls have enough compressive strength for modest buildings and gain stability from wall

thickness. Although flexural strength for adobe walls is low, traditional adobe codes strengthened walls

by keeping distances between bracing 6m or less and walls to 6 or 8 times as high as thick.

Earthbag walls are slightly thicker than some adobe walls, 15 thick/ 38 cm compared to adobe at either

11/ 28 cm or 14/ 35 cm thick. Earthbag walls weigh more per length than the thinner adobe walls, and

for structural purposes should be compared to thicker adobe.

3Brandon Ross, Wind Testing of Earthbag Wall, paper for Engineers Without Borders chapter at the University of

Florida, 2011. Accessed online 8/9/2011 at4Wall portions tested were all 1.1 x 0.963 m = 1.05 m and approximately 11/ 28 cm thick.

5Chris Croft, Structural Resistance of Earthbag Housing, unpublished Master Engineering thesis, 2011

LABORATORY TESTS COMPLETED EARTHBAG TESTING INFORMAL EXPERIMENTS

PRELIMINARY TRIALS SMALL PLASTERED WALLS PRELIMINARY TRIALSFULL SIZE PLASTERED WALLS/BUILDINGS

CONVENTIONAL EARTHBAG ALTERNATIVE GRAVEL &SANDBAG

TESTS TO DATE STRONGCOHESIVE

FIL L

MEDIUM

COHESIVE

FIL L

WEA K

COHESIVE

FILL

SANDY

COHESIVE

FIL L

STABLE

GRAVEL FIL L

SAND FIL L EXPERI-

MENTAL

FILLS

COMPRESSIVE

FRICTION

WIRE PULLOUT

FLEXURAL

SHEAR MOST NEEDED TO ADVISE

IN HAZARDOUS REGIONS

SHAKE TABLE


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Earthbags have greater potential tensile strength and flexibility than unreinforced adobe walls from

their matrix of barbed wire and poly fabric. With adobe-type soil fills, bag walls may have greater

strength than adobe because of the tamping process during construction. But earthbag units also can

use a wider range of soils, because each earthbag is protected by the confining bag surface and

mandatory plaster. More should be learned about fitting soil fill to the level of hazard at the site.

ADOBE RESEARCH AND STANDARDS

Engineered standards for earthen buildings based on structural testing of adobe and rammed earth

were created in 1998 in New Zealand.6 The non-engineered guidelines for adobe appear to rely more on

tested shear strength than on flexural or out-of-plane strength.

The New Zealand Standards include a valuable guideline for non-engineered buildings 7, assuming

standard grade quality for adobe. Those who cannot afford custom engineering or complex materials

testing have a means of checking that their building has enough bracing for low, medium or high seismic

risk regions. The design shear strength values assumed for standard grade materials are conservative.8

This code only allows unreinforced adobe walls in low seismic hazard areas. Guidelines for reinforced

adobe required in high seismic risk areas rely on vertical steel rebar and horizontal plastic geo-grid inside

un-plastered walls.9 This type of intensive reinforcement (possibly combined with stronger special

grade earth blocks) has resulted in some high shear test values,10 but creates a limited ductile response.

New data after several earthquakes in New Zealand affirmed these strength standards for adobe.11 A

magnitude 7.1 earthquake caused 0.7- 0.8 peak ground acceleration at the site of one reinforced adobe

house with 28 cm thick walls. Very limited damage resulted despite significant shaking. For ten modern

earthen houses of adobe and other earth techniques, reinforcement and design conforming to the

standards resulted in only minor damage due to differential ground movement or excessively high walls.

Evidence reviewed in the same paper also showed that some adobe without code level reinforcement

survived severe vibration well. One historic earthen cottage with a ground floor of unreinforced 20/ 50

cm thick adobe received a lower magnitude but very shallow earthquake that caused peak ground

6Hugh Morris, Getty Conservation Institute, New Zealand: Aseismic Performance- Based Standards, Earth

Construction, Research and Opportunities, in Proceedings of the Getty Seismic Adobe Project 2006 Colloquium.

Accessed at http://www.getty.edu/conservation/publications/pdf_publications/gsap_part2b.pdf, 8/8/2011, p. 607

Standards Council, New Zealand Standards 4299: Earth Buildings not Requiring Specific Design, New Zealand :

19988

Based on allowed bracing capacity of unreinforced, partially reinforced (vertical rebar at ends) and reinforced

adobe, NZS 4299, pp. 53- 559One half inch/ D12 vertical rebar every 61/ 1.65 m for an 8/ 2.4 m height wall tied to footing and bond beam,

with horizontal rebar or geo-grid every 17.5/ 45 cm minimum. Standards Council, NZS 4299: 1998 p. 5810

Standards Council, New Zealand Standards 4297: Engineering Design of Earth Buildings , New Zealand : 1998 p.

2311

Hugh Morris, Walker, Richard and Drupsteen, T. Modern and historic earth buildings: Observations of the 4th

September 2010 Darfield earthquake, Auckland, New Zealand: Proceedings of the Ninth Pacific Conference on

Earthquake Engineering, 14-16 April, 2011 accessed 8/8/2011 athttp://db.nzsee.org.nz/2011/133.pdf.
http://db.nzsee.org.nz/2011/133.pdfhttp://db.nzsee.org.nz/2011/133.pdfhttp://db.nzsee.org.nz/2011/133.pdfhttp://db.nzsee.org.nz/2011/133.pdf


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acceleration levels as high as 2.2. Although the earthen walls supported an upper story of timber

framing, they still received only relatively minor and repairable cracking within walls.

Engineers at the Catholic University of Peru have sought less expensive ways to reinforce existing adobe,

since steel rebar is costly and must be installed in new walls. They have tied interior and exterior geo-

grid layers together through holes drilled in existing adobe walls, and plastered with earth.

This retrofit system preserves the ductile performance of adobe. Shake table tests of scale buildings

have survived horizontal motion up to 1.2 g without collapse. Their test (marked APM on the following

chart) registered a peak strength of 40 kPa, more than twice as high as simple plastered adobe (AP).

Plaster and mesh reinforcement increase shear strength dramatically.

Many of the researchers tested primarily for lower serviceability limits. In the developed world builders

prefer to use standards that will prevent the need for re-plastering with gypsum materials that require

high skills after a wall deforms more than 1/250 of its height.

In contrast, this comparison focuses on higher peak strengths from the same reports. Peak strength isthe most force a wall can resist. When the mortar joints of an adobe wall crack enough to allow the

center of gravity of the wall to move 1/2 or 2/3 of the wall thickness, most become unbalanced and

overturn or collapse. In the developing world preventing wall collapse may be more important than

preventing plaster damage. Earthen or lime plasters are repairable. Plaster that cracks during vibrations

can also protect underlying walls.12

Does earthbag have enough strength to weather earthquakes like adobe?

The chart that follows compares the peak strength of test results and standards values in pressure

versus test wall size. Although peak values are given for both adobe and earthbag, the effects on these

test walls are different. Unreinforced adobe walls are usually unstable after peak strength. Adobe

reinforced with either plaster and mesh or internal rebar remain standing even though deformed.

Tested earthbag walls with only barbed wire also remained standing and mostly stable although

deformed..

Most tests were completed with dynamic cyclic horizontal pressure on tops of wall portions. Only the

CUP test reflects shake table performance of an entire building (for this one record m tested is an

approximate figure for the wall length that does not reflect total area of building walls).

12Torrealva, Neumann, Blondet, Earthquake Resistant Design Criteria and Testing of Adobe Buildings at Pontificia

Universidad Catlica del Per, p. 6: the mud plaster over the mesh greatly increases the initial shear strength and

the stiffness of the wall, controlling the lateral displacements and preventing the cracking of the wall to a great

extent.


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LEGEND

S=EARTHBAG WALL WITH NON-COHESIVE SAND FILL A=ADOBE WALL

P=PLASTERED M=MESH REINFORCEMENT IN PLASTER

V=VALUE FROM BUILDING STANDARD R=REINFORCED;REBAR &?

-R=PARTLY REINFORCED;VERTICAL REBAR @ENDS R+=REINFORCED (VERTICAL REBAR,HORIZ.GEO-GRID)

X=APPROXIMATE RESULT CHANNELIZED SANDBAG SYSTEM

Peak strength shear values from tests of sand bag walls so far have not approached the high shear

values seen in plastered adobe with plastic mesh or in steel reinforced adobe. They have been near or

below the conservative standard grade shear strengths assumed by NZ for non-engineered buildings.

With cement stucco and chicken wire sandbags approach the strength assumed for unreinforced adobe.

Can earthbag with cohesive fill approach or surpass the strength of plastered and geo-grid strengthened

or heavily steel-reinforced adobe?


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SUMMARY OF TEST AND STANDARDS VALUE DATA

UNREINFORCED EARTHBAG UNREINFORCEDADOBE

PARTLY REINF. REINFORCED ADOBE

Flex Shear Strength

ES

(sand)

Croft *

(sand)

Morr

is

NZ Standards

values

CUP

**

Walker &

Morris

NZS

Valu

e

Barbed

wire

- - Barbed wire

- - - - Vert.rebar

- Horizontal & vertical

rebar

Plaster &

mesh

+

Wire

mesh

+

Wire

mesh

- +

Wire

mes

h

No plaster +

Plastic

mesh

No plaster

Panel

size

82

sf/7.7

m

26/

2.4m

11 sf/

1 m

11

sf/ 1m

11

sf/ 1m

61/

5.76m

2.4

htmin

1.5 L

2.4 ht

min1.2 L

Un-

kown

24/

2.2m

1

34/

3.2m

1

2.4

htmin

1.2 L

Failure

Strength

44/

2.1

23/

1.1

50/

2.4

141/

6.8

225/

10.8

194/

9.4

39/

1.9

134/

6.4835/

40

188

0/

90

135

8/

65

169/

8.1

Design

Strength1

16.1/

0.77

35/

1.7

99/

4.8

158/

7.6136/

6.6

27.2

/ 1.3

94/

4.5

585/

28

1316

/ 63

951/

46

118/

5.7

*All tests were on 80% size bags. These results have been multiplied by 1.25 to approximate values for a full

width wall

**These values were for scale building of unspecified size. No data on wall thickness.

Note: All designers use factors of safety to insure that there is a margin of error to prevent damage to

buildings. The NZS committee recommends a design factor at 70% of the failure value for reinforced

walls. Test failure values and design strengths were adjusted by this proportion to provide fair

comparisons.


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MATERIALS

To begin to answer these questions, the author built a 4 x 4/ 1.2 x 1.2 m height earthbag wall panel in

May 2011. In late July it was subjected to a simple form of diagonal compression.

This test used a low-strength cohesive soil as fill to establish a lower boundary for

unreinforced earthbag wall shear strength. Builders using cob and adobe

recommend minimal silt in building materials,13 and engineers assume that silt is 1/3

as strong as a mixed sandy-silty clay.14 But acceptable parameters for successful

earthbag building are not yet known.

The soil used was mostly a material called silt by a local gravel yard, with about 20%

by volume of a moderate strength clay added. The clay contained both fine and very

coarse sand with some very fine gravel. The traditional earthbag builder sediment

test15 revealed a profile of more than half silt, and less than 10% clay. A soil sample

has been retained for more accurate analysis.

Right: View of settled soil sample

Preliminary bag tests confirmed that the tamped, cured fill became firm and held a

3 nail securely. This fill created a stable wall, but individual units had lower strength than required for

adobe. Unlike informal tests for adobe strength, when dropped from 3 (90 cm) height some earthbag

units with this fill broke in two pieces or lost portions larger than 4 (10 cm).16

Bags used were standard, new woven solid poly fabric 18 x 30 inches (46 x 76 cm) which produced a

conventional 15 inch/ 38 cm thick wall. Two strands of low-tensile strength galvanized barbed wire were

used between each course.

A clay-rich earthen plaster with manure for fiber was

applied as a base coat to fill the characteristic nooks

between bag courses. One side was finished with a

finer earthen plaster containing chopped straw. The

other side received a standard lime plaster in one coat

with a small proportion of chopped straw.

Right: Trampling chopped straw into finish plaster

13Lynne Elizabeth and Adams, Cassandra,Alternative Construction, NY: John Wiley & Sons, 2005, p. 99

14NZS 4299 p. 25

15Kaki Hunter and Kiffmeyer, Donald, Earthbag Building, Gabriola Island BC: New Society Publishers, 2008, p. 15

16Standards Council, New Zealand Standards 4298: Materials and Workmanship for Earth Buildings, New Zealand :

June 2000, p. 64


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PROCEDURE

WALL CONSTRUCTION

A 5-6/ 1.7 m long wooden base was constructed of pressure treated Douglas fir assembled with

galvanized deck screws. 2x8/ 50x200 lumbers formed the sides and 2x12s/ 50x300 formed the top and

bottom plates. 4 pieces of blocking were includedinside to support the wall weight. Galvanized metal

nailer plates were added above the axle holes and at

the ends of the boards for reinforcement.

Right: wooden base

Right below: Metal plates at axle location

This base was placed on the gravel of a firmly

compacted parking area and leveled.

The first course of doubled gravel bags was placed

down onto nails sticking up from the base. This

course was 58/ 1.47 m long. Additional nails were

hammered in at each end of the gravel bags. The

gravel bag course was tamped lightly. A second layer

of bags contained very coarse sand. This measured

50 long and was tamped firmly.

The earthen fill was mixed by inter-layering silt with

dry clay crumbled by hand into 1/ 2 cm or smallerpieces. Soil was hand mixed in wheelbarrow loads,

and moistened enough to break into 2 or 3 pieces as

1.5/ 4 cm balls dropped from 5/ 1.5 m height.

Right: Weeper bag

Because of the high silt content, soil moisture content

was hard to regulate perfectly. No drier batches of

mix were used, but many batches were damp enough

to cause weeper bags. These bags ooze moisture as

tamped. It is noted in Earthbag Building17 that these

types of bags with clay fill cure stronger than drier

bags. With a silty fill these bags are too bouncy to

compact under tamping unless dried for a day first.

The damper silty bags were not obviously stronger.

17Hunter 2008, p. 19


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Right bottom: Minimal bag overlap

Building short earthbag wall portions is more complicated than

longer walls. Bags at each end of the wall must be pre-tamped well

during filling. Small end bags should be at least 12 long/ 30 cm.

Although running bond bag overlap is considered best at 8/ 20 cm

or longer, it is difficult to make 24/ 60 cm long bag units with the

common 29 or 30 long bags. Custom longer bags may be helpful or

a longer test wall length. On this 42/ 1.27 m long wall, running

bond overlap ended up as small as 4/ 10 cm on many courses.

During tamping each bag course reduced in size from 7 to 5/ 18

to 12.5 cm and became firm. Bag courses also keyed into the

course below and settled around the wire barbs during tamping.

Right above: Before and after tampingRight: Finished test wall

10 courses total of bags were placed, with sides and ends plumb. .

The wall was covered with a tarp in rainy weather. After 2 days the

sides of the wall were tamped with a mallet to flatten the wall

surface. When the upper course was tamped an adult sat on the

bag layers to prevent wall shaking and bag movement.

Within a week earthen infill plaster was placed in the nooks

between courses on both sides of the wall. The plaster was appliedto the bags when the bag fill had become firm but was not

completely dried.

This plaster consisted of moderate strength clay with a small

amount of added sand and manure. Several days later a single coat

of lime plaster was added to one side and an earthen finish coat on

the other. The lime plaster was 1 part lime putty to 3 parts sand

with added chopped straw. Thickness of finish layers varied from

0.50- 1 inch/ 1.5- 2.5 cm. Bags were left exposed at the ends of the

wall.Right above: Lime and earthen plaster side view

Right: Lime plaster condition when cured

These finish surfaces were kept covered with a tarp and sprayed

lightly with water twice a day during curing. After two months of

curing the finish plaster coats were well dried. The lime plaster

dried with 4 minor non-structural cracks less than 1/8 inch/ 3 mm


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wide. The longest crack was 10/ 26 cm long. The earthen

plaster had more cracks and wider, some up to a quarter

inch/ 6 mm across. All the earthen plaster was well anchored

to the wall, and fibers were visible spanning the cracks and

uniting the plaster.

Testing occurred 2 months after plastering. At the time of

testing the lime plaster and most of the finish earthen plaster

was dry. Because of leaking through the tarp covering the

wall portion, the earthen finish plaster at the upper corner

where the diagonal pressure was applied was damp.

Right top: Earthen plaster crack close-up

Below: Earthen plaster condition at time of testing

TESTING EQUIPMENT

An upper corner bracket was attached by chains to a lever

arm on the opposite side base. The far end of the lever arm

was attached to a rope that led through a single pulley to a

support for weights.

Right top: Overall view of testing

Right: Corner bracket

The plywood corner bracket was capped by a piece of angle

iron. Chains looped over and ran down to turnbuckles.

The lever arm was a single 2x10/ 50x250 of pressure treated

Southern Yellow pine. Metal brackets were screwed to thewood to seat the lever arm in place on a pivot point formed

by an axle of metal pipe. The turnbuckles were attached to a

strong piece of aluminum channel bolted to the other side of

the lever. The turnbuckles ran through eye-bolts held in place

with large washers.

Right bottom: Chain attachment to lever


12/16


The chain was attached to the lever arm 5/ 13 cm above the center of the axle on which it rotated. A

rope was tied to the other end of the lever arm exactly 113/ 2.87 m from the axle. This produced a

mechanical advantage of 22.6 to 1.

Right top: The laser dot at the beginning of testing

A strong nylon was used for the rope from the lever

arm to a pulley suspended by ropes in line with the

wall. Force was applied to the lever arm by placing

gym weights on a shelf of wood and metal strapping.

A vertical line was marked on the finished plaster

surface in pen from the top of the wall to the bottom.

At the level of the second course from the top a thin

plastic metric ruler was taped and nailed to the

plaster with brads. A contractors laser level located

4/ 1.2 m distant was aimed at the ruler.

Right: Lever arm and rope angle

TEST

Weights were applied, left in place for 2- 3 minutes,

and then removed, in gradually increasing

increments. Measurements were noted and

photographs taken after each increase.

When the first 50 pound/ 23 kg weight was first

applied, the rope stretched. The weight was removed

and the support shelf raised higher from the ground.

Three times the wall withstood the force applied by

100 lbs./ 46 kg with only 0.3 inch/ 8 mm displacement

and little plaster cracking. The lever arm formed an

angle between 90 and 103 degrees with the rope

during tests.

Right: Cracking after 2nd application of 100 lbs.

After the first two applications of 100 lbs./ 46 kg

weight (2260 lbs.-force or 10 kN) the wall flexed back

to 0.1 or 0.16 inches/ 3 or 4 mm displacement.

During the course of testing the very thin cracks first noted in the lime plaster spread. By the third

application of 100 lb./ 46 kg weights cracks had nearly joined and stretched from the upper corner

bracket down to within about 24 inches/ 60 cm of the base of the wall near the chain.


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The chain rubbed on the plaster for several inches near the bottom corner of the wall. No cracks

appeared in this region.

Right: Friction between chain links and plaster

After the third application of 100 lbs./ 46 kg weight (2260

lbs.-force/ 10 kN) the wall remained at 0.3 inches/ 8 mm

displacement. Turnbuckles were turned and chain links

removed to lessen slack in the chain.

At 120 lbs./ 54 kg weight (2712 lbs.-force/ 12 kN) the wall top

moved a total of 0.4 inches/ 10 mm and did not recover

when the weight was removed. Each time when more than

120 lbs./ 54 kg was placed on the weight shelf, the wall top

moved enough to cause the shelf to bottom out.

At this force a second major crack developed in the limeplaster above the level of the chain. On the earthen plaster

side the most severe cracking was above the chains and

obviously stair-stepped.

Right above: Lime plaster after 3rd try of 100 lbs./ 46 kg

Right: Earthen plaster after 3rd try of 100 lbs./ 46 kg

Peak strength may have been slightly more but a small

enough increment was not used to be accurately measured.

After 4 repetitions of more than 120 lbs./ 54 kg weight, theupper plaster was still firmly adhered to the bag wall.

Although there were large cracks and some plaster surfaces

overlapped, no plaster fell off the wall.

Right bottom: Final condition of lime plaster

The upper plaster was next removed to allow clear view of

the relative motion of the underlying bags.


14/16


-5

0

5

10

15

0 5 10 15 20

kN

mm

Earthbag Wall Displacement per

Force(Weak Cohesive Earthen Fill)

DATA

RESULTS

The earthbag wall portion displayed some elasticity until

peak strength was reached.

After three applications of more than 120 lbs./ 54 kg of

weight, the upper courses of bags shifted horizontally a

total of 10 inches/ 25 cm. Unlike the Bath University tests

on sand-filled bags, the bags did not move apart during

the test.18

The wall end far from the applied pressure also

compressed about 2 inches/ 5 cm, 4% of its height.

Left top: Bag wall with plaster removed

The earthbag wall was not significantly less stable than

18Croft, p. 43

RESULTS DATA

Weight

(lbs.)

Lbs.-

forcekN mm

50 1130 5.02 0

100 2260 10.05 4

50 1130 5.02 4

0 0 0 3

50 1130 5.02 3

75 1808 8.04 4

100 2260 10.05 7

0 0 0 4

50 1130 5.02 4

70 1582 7.04 5

80 1808 8.04 6

90 2034 9.05 7

100 2260 10.05 8

0 0 0 7

120 2712 12.06 10

0 0 0 10

100 2260 10.05 10

120 2712 12.06 10

>120 >2712 >12.06 12+


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before the test. Out of plane pressure by hand did not seem to flex the wall more easily than before

testing.

Right top: Barbed wire openings

The most significant mechanism causing horizontal

wall deformation was evident when the upper two

courses of bags were examined. The poly bag fabric

was slit only slightly at the barbed wire entry points.

The contents of the bags were still in firm blocks.

But when the fabric was removed from two bags in

the second course to examine the earthen fill, each

wire barb had flaked loose a conical portion of the

bag fill material. Some slight cracking in the surface

layers was visible between the barb attachment

points also. The surface of the earthen unit distantfrom the barb attachment points was firm and

undisturbed.

Right: Bag fill loosened at barbed wire points

CONCLUSIONS

Despite the use of very simple testing equipment, this small experiment produced some new data.

Peak loading on this 15 sf/ 1.4 m panel of plastered earthbag under static loading was about 2700 lbs./

12 kN. Considering the panel size, it resisted 175 psf/ 8.4 kPa. Silty soil bag fill with barbed wire but no

reinforcing mesh is very close to the 8.6 kPa value of the sandbags with cement stucco and chicken wire.

More testing with superior measurement systems is needed to evaluate how close this preliminary

experiment comes to a valid indication of the shear strength of earthbag walls. This informal static trial

of a weak cohesive soil earthbag panel may not be exactly comparable to the dynamic sand bag or

adobe tests. Dynamic testing would be preferable since it often returns values higher than those

measured in static testing, and may more closely approximate seismic performance.

This earthbag wall was purposely built of low-strength materials, to determine if ordinary earthbag walls

built without special fill could be as strong as unreinforced adobe. This wall did not contain the vertical

rebar that is usually inserted near openings in earthbag, which should also raise strength values when

used in strong cohesive soil fills.

How does earthbag compare to the required and tested strengths of adobe?


16/16


LEGEND

S=EARTHBAG WALL WITH NON-COHESIVE SAND FILL A=ADOBE WALL

P=PLASTERED

Shear strength for earthbag depends on the bonds that can transmit the tensile strength of barbed wire

to the earth units and their surrounding fabric. Strong clay fills, like those used in adobe, when tamped

become strong enough that barbed wire barbs will bend back rather than gouge or flake the bag fill

under stress.

Alternative reinforcements include small chunks of mortar added to barbed wire at depressions notched

between bags, returning barbed wire to higher courses at openings, and the addition of some simple

wire mesh pins.

Earthbag is inexpensive, easier to learn to build than adobe, and is already used in a wider range of

climates than adobe. But most importantly, it deforms without collapsing under severe stress, offeringthe potential to save lives even if buildings are damaged.

If this hybrid earthen, poly, and wire material is strong enough for moderate seismic risk zones without a

lot of added rebar and cement, it will be a great service to those rebuilding after disasters. Its flexibility

could possibly make earthbag the best way to build cheaply and sustainably in high seismic risk zones.

Shear Strength of Earthbag Wall With Weak Cohesive Fill

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